Field of the Invention
The present invention relates to direction finding of radio frequency signals, and particularly to a system and method utilizing a reconfigurable antenna to estimate signal direction.
Description of the Related Art
Conventional radio direction finder systems are typically phased array systems. Phased antenna array systems are based on a fixed antenna pattern of each antenna element in the antenna array. Arriving signals from different antennas are weighted to create proper beam forming in the direction of arrival. A phased array depends on the antenna geometry and whether the particular antenna array can estimate both angles (i.e., azimuth and elevation). For example, a horizontal linear array can be used to estimate the azimuth angles but not the vertical angles, and the reverse is true for a vertical linear array. The planar, circular, spherical or alike antenna array configurations may measure both the azimuth and vertical angles. The size of conventional phased antenna array depends on the operating frequency of the signal of interest, since the spacing between antenna elements is related to the wavelength of the operating frequency. Direction finding over a wide operating frequency range requires different sizes of antenna arrays, which is typically not convenient due to the actual physical implementation of such systems and/or degradation in performance if proper requirements between antenna space and the wavelength of the operation frequency are not met.
Conventional antennae typically have a fixed radiation pattern at a specific operating frequency and bandwidth. Reconfigurable antennae, however, have the capability of dynamically changing their characteristics, such as radiation pattern, polarization and operating frequency. The reconfiguration of such antennae can be achieved via different techniques, such as altering the physical structure of the antenna, altering feeding methods, controlling current density and the like. Ultimately, the choice of reconfiguration method is based on the design requirements and performance level required. The distribution of current in an antenna and its geometry determines how the antenna radiates its energy into a radio channel, or how it receives radio frequency energy from it. The complex far field radiation pattern of an antenna can be mathematically expressed as:F(θ,φ)={circumflex over (r)}×[{circumflex over (r)}×[∫V′JV′(r′)e−jβ{circumflex over (r)}r′dν]],  (1)where F(θ,φ)=(Fθ(θ,φ),Fφ(θ,φ))εC2×1, JV′(r′) is the current distribution in the antenna, {circumflex over (r)} is the unit vector in the direction of propagation to an observation point, and r′ is a vector from the coordinate system's origin to any point on the antenna.
From equation (1), it can be seen that by changing the antenna's physical configuration r′, the current distribution JV′(r′) will change, which is reflected by altering the complex far field radiation pattern F(θ, φ). Thus, controlling distribution of the current in the antenna, JV′(r′), leads to control of the radiation pattern F(θ, φ). Thus, the overall goal in using a reconfigurable antenna is to control change of the current distribution around the antenna, which leads to altering the radiated far field. Such changes can be achieved by modification of the antenna geometry or its material properties.
The implementation of reconfigurable antennae can be accomplished in three different processes, namely a design stage, a simulation stage, and an optimization stage. The antenna design stage includes selection of the radiating structure of the antenna and its reconfigurable aspects. The selection process is based on several performance parameters, such as power consumption, directivity, bandwidth operating frequency and the like, in addition to design constraints, such as antenna size, fabrication costs, etc. These antenna performance parameters have to be fulfilled for every antenna state to ensure that the reconfigurable antenna can work as expected when it switches from one antenna state to another.
One antenna structure that is known for its versatility in many applications is the patch antenna. These antennae can be very small in size, making them attractive for numerous applications, since they can be arranged in different geometries (rectangular, circular, dipoles, etc.). The selected geometry depends on application and performance parameters. Patch antennae can be used in arrays to adapt to different radiation patterns, polarization and operating frequency, thus making them desirable for use in reconfigurable antennae.
Following selection of antenna structure, it is important to select the choice of reconfiguration; i.e., how reconfiguration will take place. Antenna reconfigurability can be categorized into four different reconfigurability functions, namely 1) reconfiguring resonance frequency, which usually takes place by changing physical properties that alter surface current distribution; 2) reconfiguring radiation pattern, which usually takes place by changing radiating edges, slots, or the feeding network; 3) reconfiguring polarization state, which usually takes place via changing the surface structure or the feeding network; and 4) combinations of reconfiguring the above characteristics, which usually takes place by using numerous techniques simultaneously. It is very difficult to configure frequency, radiation pattern and polarization independent of one another, as changing one characteristic will change the others. Thus, careful design and analysis are very important. As such, it would be desirable to be able to use the fourth reconfiguration function; i.e., combination of multiple techniques, such as simultaneously.
With regard to signals and antennae, the field of direction finding involves measuring and evaluating signal strength to find the line of bearing from a signal source to the direction finding system (commonly referred to as the “angle of arrival”). The direction of arrival of signal propagation from a radio source can be defined by two angles: the azimuthal angle and the elevation angle. The azimuthal angle is typically measured relative to North, although other reference directions may be used. The elevation angle can be measured relative to the horizon or the z-axis relative to the coordinate frame of the direction finding device.
Thus, a reconfigurable radio direction finder system and method of using the same addressing the aforementioned problems is desired.